The present invention relates to a circuit for driving a laser diode, and more particularly to a circuit which may stabilize the power of the laser diode beam.
The power of the laser diode beam has a temperature-dependent characteristic. In order to prevent the variation of power characteristic and to stabilize the power of beam, the beam power is detected and fed back to control the current flowing in the laser diode.
Meanwhile, recently, a laser-beam printer employing a laser diode has attracted considerable attention. In a laser-beam printer, an optical device is positioned to cause the laser-beam to scan each dot line on a light-sensitive drum. At a start point of the scanning which is located outside the drum, the laser diode is excited to provide a timing signal. When the laser-beam reaches an initiation point, a sensor detects the laser-beam to produce a signal. By the signal, the laser diode is turned off. When the laser-beam reaches a predetermined starting point on the drum and there is at least one dot to be printed on the dot line, the laser diode is intermittently excited in accordance with a command signal to start to scan the dot line from the starting point. However, if there is no dot to be printed on the dot line, that is the dot line is blank, the command signal is not produced, and hence the laser diode is not excited after the starting point. In such a case, although the laser diode remains off, the optical device has operated through one cycle.
However, in order to stabilize the power of the laser diode beam, it is necessary to store a value of the beam power at the instant cycle so as to produce the same beam power in the next cycle.
Thus, when a dot line is blank, the power of the beam at the beginning operation of the scanning must be stored until the beginning of the next cycle, since the laser diode is not excited after the initiation point. For this purpose, a sample-and-hold circuit is usually employed.
However, the sample-and-hold circuit has a complicated structure. Further, if the timing for producing sampling pulses does not synchronize with switching pulses for the laser diode, the laser diode malfunctions.